Noise pollution in our cities is an increasingly serious problem. Experts are now beginning to realize that in addition to the physical damage sounds can do to hearing, loud sounds have several other physical effects. Sounds in the range of 140 decibels--the level that is reached a few feet from the catapult of an aircraft carrier--produce a number of unpleasant bodily sensations: a feeling of vibration inside the head, severe pain in the middle ear, loss of equilibrium, and nausea. Certain low frequency sound is likely to produce a startle reaction wherein norepinephrine and other body hormones are released for the purpose of preparing the body to respond to an emergency. The startle reaction causes the blood pressure and the pulse rate to jump, the muscles to contract, perspiration to increase, the flow of saliva and gastric juices to reduce, and digestion to cease.
Noted experts in the field of sound, such as C. P. Boner claim that the sound to which urban dwellers are exposed contribute to circulatory troubles, loss of hearing, fatigue, and emotional disturbances. In particular, others in the field believe that the average urban dweller is subjected to stress, particularly because he is unable to get prolonged, uninterrupted sleep. In fact, urban street noise has been measured to range between 80 decibels on the quieter main streets in the smaller cities, to a maximum of approximately 109 decibels a few feet from an accelerating diesel bus.
The home itself is, of course, increasingly an independent source of sound, particularly from such appliances as dishwashers, garbage disposals, and vacuum cleaners. These appliances create noises in the vicinity of 60 to over 100 decibels.
It has long been known that it is possible to fight noise with white noise. White noise is often compared to the soft rush of escaping steam. White noise is often used to convert disturbing silence into controlled quiet as well as to mask noises which would otherwise by distracting. White noise is now commonly used in dentistry to mask nervewracking drilling noise thereby removing much of the tension, anxiety and pain associated with the drilling operation.
The need for effective noise elimination or masking is quite pervasive. Psychologists Sheldon Cohen, David Glass, and Jerome Singer claim that young children may suffer impaired learning ability due to the continual background din from their living close to a busy highway which makes it difficult to discriminate home conversations, and therefore, their understanding of words. According to the National Research Council of Canada, 35-million Americans live in areas where 24-hour noise averages 65 decibels--annoying if not necessarily harmful-- and 90% of our population is subject to sporadic and "intrusive" sounds that exceed 75 decibels. The World Health Organization estimates that noise costs the United States 4-billion dollars annually in accidents, absenteeisms, and compensation claims.
Of course, in addition to the loudness of the particular sounds, it has been speculated that those sounds which intermittently interrupt everyday activities may in fact, be the most harmful. Pschologist Howard M. Brogard has theorized that the most harmful noises are those that invade one's dreams, because dreaming is necessary for mental health. When the dream is interrupted by a jet plane, a fire engine, or a noisy neighbor, the person will dream twice as much the next night. But if he is interrupted again and again, he will become emotionally upset. It has been speculated that this is the explanation for the relatively high admittance to mental hospitals which has been found for residential areas surrounding airports such as Heathrow Terminal outside London. It is theorized therefore that people will actually sleep better when subjected to heavy but regular traffic outside their windows than when the traffic is lighter but unpredictable.
Although the obvious answer to the noise pollution problem is to reduce the noise to which we are all subjected, the Environmental Resources Agency contends that it will take until the end of the century to shave five decibels off the sonic environment. It has been estimated that to lower the industrial noise level by five decibels would cost from between eight and thirty-one billion dollars. These and other problems relating to the area of noise pollution are discussed in S. S. Stevens and Fred Warshofsky, SOUND AND HEARING, 1965, Time-Lift Books, Inc., New York; New York Times Magazine, VI (Nov. 23, 1965), "Noise," by David Dempsey, pp. 31ff; and PHYSIOLOGY AND BIOPHYSICS, edited by T. C. Ruch and H. D. Patton, 1965, W. B. Saunders and Company, Philadelphia, 19th edition, Chapter 18, pp. 379-384.
It has long been known that one sound may be obscured or masked by another. The most commonly used masking stimulus is white noise, noise with a uniform power spectrum from one extreme of its frequency range to the other. It is also known to attempt to directly mask a tone by producing a second signal relatively close to that tone. In this case, beats are produced between the frequency of the primary and the frequency of the masking tones unless those tones are of too short a duration to permit a full cycle of beating to occur or, alternatively, if a narrow band of noise rather than a tone is used as a masking sound. Although it has been suggested that there are critical bands for masking tones which act to activate a particular filter response in the ear which preempts a particular band width and therefore, recognition of the sound to be masked, these theories are not yet definitive, particularly in view of the widely recognized phenomenon of remote masking. First discovered by Bilger and Hirsh in 1956, remote masking is clearly present when high noise levels such as 60 to 80 db are used to mask frequencies below those of the band of masking noise. For example, a band of noise from 2450 to 3120 Hertz (Hz) at a spectral level of about 70 db will elevate the threshold for tones from 100 to 1000 Hz by about 20 db. Although not yet definitively understood, other factors which appear to play a part in masking are temporal qualities, such as the duration between the occurrance of the masking tone and the tone to be masked (either forward or backward), the distribution of the noise and noise plus signal, the energy band width and duration, and the signal or frequency uncertainty of the masking tone. A complete discussion of these and other masking criteria is found in FOUNDATIONS OF MODERN AUDITORY THEORY, edited by Jerry V. Tobias, 1970, Academic Press, New York, Chapter 3, "Masking," by Lloyd A. Jeffress, pp. 87-114.